Radiofrequency choke coil system



A. cR'ossLEY RADIOFREQUENCY cHoxE COIL SYSTEM Filed July 20, 1925JJJOJJJJ @www www@ XXXX #XXX 452/ /864 TH..

May l0 1927.

- INVENTOR. ,d @was e A TToRNEY Patented May 10, 1927.

UNITED STATES PATENT OFFICE.

ALFRED CROSSLEY, F WASHINGTON, .DISTRICT 0F COLUMBIA, ASSIGNOR TO WIREDRADIO, INC., 0F NEW YORK, N. Y., A CORPORATION OF DELAWARE.

RADIOFBEQUENCY CHOKE COIL SYSTEM.

Application led July 20, 1926. Serial No. r123,785.

in which a plurality of choke coils consti4 tutes the system, each chokebeing designed to be equal in resonant frequency condition r to that ofthe other coils for securing maxi` mum impedance over a broad band offrequencies. f

A further object of my idnvention 1ste provide a choke coil systemhaving a greater inductance and a lessv distributed capacity than it hasheretofore been possible to obtain.

Still another object of my `invention is to produce a choke coilarrangement which will have greater impedance over a broader band offrequencies than it has heretofore been possible to obtain with anyknown 'single choke coil.

My invention will be more clearly understood from the followingspe'ciication` and by reference to the accompanying drawings, in which:

Figure lA is a graph of the characteristic reactance' curve of a chokecoil as used in the circuit arrangement of a high frequency oscillatorsystem; Fig. 2' is a side elevation illustrating one arrangement of themultiple choke coil system of my invention; Fig. 3 is a graph showingthe characteristicl reac- 0 tance curve of the multiple Vchoke coilsystem illustrated in Fig. 2; and Fig. "L shows a modified arrangementof choke Vcoils em ploy-ed in the system of my invention.

I have found that a single layer, banked wound or pancake type of chokecoil is limited from a high impedance standpoint to a narrow band offrequencies. I have also observed in the operation of high frequencyelectron tube oscillator circuits that these types of choke coilspossess certain qualities that render them dangerous in operation atcertain fr uencies. Many cases have been recorded W ere the coils'wereactually destroyed-dueto their low impedance at certain frequencies.-Tests'show that single choke -coils have very low impedance values atfre quencies slightly less than two, four, or six times theirfundamental resonant frequency.

Inasmuch as single choke coils are very seldom operated at frequenciesgreater than twice the resonant frequency of the coil it will be best toconsider their characteristics over a range from one-half to twice theresonant frequency of the coil. Considering the choke coil from animpedance standpoint, all choke coils are parallel electrical circuitshaymg inductance and capacity, the capacity being that of theJdistributed capacity of the coil. This distributed capacity is thesummation of the capacities between turns, be-

tween inner and outer layers and that capacity which exists between thecoil and grounded objects. This condition exists in the universal woundor pancake types of coil, which types are particularly suited for chokecoil use.

The parallel combination in the choke coil therefore permitsmaximumimpedance to currents of a frequency equal to that of \the resonantfrequency of the coil, and a decrease in impedance for frequencies aboveand below the resonant frequency. Upon closer study of the choke coil,especially when it is considered that one end 'of ther coil is always atground potential, it will be Y noted that it has characteristics thatresemble those of an antenna. An antenna has maximum capacity andinductance at its base and minimum inductance and capacity at its top. Achoke coil grounded at one end,

preferably the outer end in the universalv wound type, has maximumcapacity and inductance at this outer end and minimum inductance andcapacity at the inner terminal.

Again comparing the choke coil with the antenna, it will also be foundthat it vhas resonant conditions other than that of theillustrated byreference to the reactance,

curve shown in Fig. 1. In this figure, the

numerals along the frequency line or abscissae indicate the multiuplefrequency rela.- tionship; in other Words, l is the fundamental or firstfesonant frequency period of the choke coil While 2 represents doublefrequency and so on.

By reference to the re'actance curve in this figure, it will be notedthaty at the first, third and fifth frequency points, par

allel resonance is obtained.- This condition of parallel resonanceindicatcs'inaxinnim impedance and therefore for best choking ef feet itis necessary to operate the choke coil at or near the point of parallelresonance.

Again referring tothe reactance curve, it

will be'observed that at the second'and fourth multiple frequen'clesthere is a series resonant condition. These conditions are the dangerpoints in a choke coil. In other Words, under these conditions thereactaneei is Where Lv is the inductance of the coil and It ing equal toSovis the circulating resistance of the circuit.

In View of the conditions obtaining in a single choke coilit istherefore good practice to operate the coil over a range Vof frequenciesfromone-half to one and twothirds times its resonant frequency.

Having thus explained the characteristics of a singlechoke coil, I willnow describe my invention-wherein I employ a number of choke coils andthereby obtain greater impedance over a broader band of frequencies thanis obtainable with any single type clioke coil.-

The first type of choke `coil that I employ consists offtwo or morecoils of identical characteristics placed at such-distance from eachother that there is no appreciable magnetic or electrostatic couplingbetween them, and these coils are connected in series with each other asshown in Fig. 2. In this iigure, 1, 2 and 3 are the coils that areattached to a bakelite supporting rod 4. This rodis -maintained in afixed position by the supporting members 5.

The electrical characteristics of" such choke coil assembly mayloe-described as fol' lows: It may be'. assumed that the resonantfrequency of each coil is equal to the fundamental frequency-1 shown inFig. l. In-

. 'asmu'ch as there are three coils in series with no magnetic couplinobetween theril,V the total inductanceof he combination must be threetimes that of one coihwhile Vthe times that of one coil and inasmuch asthe inductance-capacityproduct is the same, the resonant frequencycondition is equal to that of one coil.

To further illustrate the reactance values, the effect of three coils ascompared with one coil may be seen in Fig. 3. In this figure tworeactaucev curves as shown, the dotted line curve indicating thereactance of the single coil over the frequency' range, While the fullline curve indicates the reactance of the three coils in series.

The operation of these three coils at the series resonant frequencies,second. fourth, and higher, will not be dangerous as the operation of a.single coilas the reactance, and consequently .the resistance at any ofthese frequencies, will be at least three times as great as that of onecoil. Under practical operating conditions I have found that theimpedance at/or near the even multiple frequencies is even greater thantheoretical conditions would indicate.

I therefore obtain With my multiple coil arrangement a .greaterimpedance over the same frequency range and have secured such anincrease in the impedance at the danger points as to render the chokecoil usable at these points. This maybe appreciated when it is pointedout that the resistance atlthc danger point for one Well designed coilis very seldom less than 10,000 ohms.

It becomes necessary under sp'eciaLci-cumstances to reduce the size ofthe choke coil and for this reason-the condition of zero couplingbetween coils may be dispensed with.' To moet this need I may arrangethe coils and place them in such position that magnetic andelectrostatic coupling between coils is effected.

This modified arraiigement produces a choke coil that has a lowerfundamental frequency than that of one coil, but still has a highimpedance overa broad band of frequencies; Suclna choke coil system isshou'n in ,Fig 4. The arrangement'of the coils permit-s the mutuaiinductance of the coils to add to the inductance of the three coils, i.e., the inductanee of this arrangement 1s approximately quadruple theinductance of one coil. The capacity of this arrangement is notone-third the capacity of. one coil but is .only slightly lower thanthat of one ooit This combination produces a choke coi'lof largeinductance and small capacity, a condition that cannot beobtained withany single coil, particularly when relatively large size Wire isemployed.

This type of choke coil has at least twice the inductance andtwo-thirds'the capaclty of the best type commercial choke heretoforeknown to the art, with the additional features of three Vtimes theimpedance at the Adanger frequencies,v e A It is important that 'collsof the s'ame sizeA be used throughout my arrangement. If different. sizecoils are employed additional series resonance conditions or dangerouspoints will be obtained, at frequencies half Way between the fundamentalresonant periods ofthe respective coils. This is particularly true whentivo coils are employed.

Referring to Fig. 4: in detail, reference character 6 represents abakelite or glass rod that-fits into a brass base or socket T. The coils2 and 8 slide down over the rod (i and are separated from each other byba'kelite or glass separators or bushings 9. A securing piece 10 fitsover the top of rod and by means of. screw 1l holds all the coils inplace. A metal connector l2 under screw 1t acts as a connector for thehigh potential terminal of the choke coil 1. The choke coil l-is fixedin position on rod 6 betiveen the upper bushing l9 and the retaining'capl0.

The coils l, Qiand 3 are connected as shown,

Vthat is, the outer terminal of coil l to the inner termina-l of coil 2,and the outer terminal of coil 2 to the inner terminal of-coil 3. Theouter terminal of the last. coil 3. connectsdirect t0 the lovv potentialterminal 14 on socket 7. his terminal is madeffast to the brass base 7by a screw l5. For in sulating the high voltage direct current source,`a porcelain or pyrex insulator lt is employed. Connection is made tothc in. sulator by the threaded end of the base 7 that screws into thevinsulator.

The special relation of the coils l, 2 and 3 may be properly selectedfor securing the .mostl desirable results and the relation permanentlyfixed for the band of frequencies over which the oscillator is designedto be operated.

'has been illustrated in Fig. 3.

The action of the plurality of choke coils The reactance characteristicof the plurality of coils connected in series shows a substantialincrease over the working frequency range as compared to the reactanceof a single choke coil. At the same time, the resistance has beenincreased at. least three times over that of a single choke coil therebyeliminating the tendency of the choke coil to be destroyed at resonantfrequencies. The fact that the individual coils are series connected inthe particular manner shown, substantially reduce's the distributedcapacity.

While I have described my invention in certain preferred embodiments, Idesire that it be understood that modifications may be made and that nolimitations upon' my invention are intended other than are imposed 'bythe scope of theappended claims.

lVhat I claim as new and desire to secure Vindividual choke coils eachdesigned to have small distributed capacity and each arranged withrespect to another with substantially negligible magnetic andelectrostatic coupling therebetween and a series connection betweensaidcoils, the outer terminal of one 4coil being connected with the inner'terminal of a. succeeding coilivhereby the reactance ot' .Laid coilsover a selected band of tref quencies is relatively large while thedistributed capacity of said coils is relatively small.

3. A choke coil system comprising a plurality of individual choke coilunits each arranged' to have relatively small distributed capacity,means for mounting said units with respect to each other in such mannerthat there is no appreciablefmagnetic or electrostatic couplingtherebetween, a series connection between said units, a. high potentialterminal located at the inside of one, of said unit-s anda` lowpotential terminal located at the outside of another of said units,said' lastmentioned units being sepa `ated from said first mentionedcoil by at least one other of said coils.

4. A choke coil system comprising in combination a plurality of chokecoil units each arranged to have small distributed capacity,

means for mounting said choke, coil units with respect to. each other insuch manner that there is no appreciable magnetic or electrostaticcoupling therebetween, a series connection between said plurality ofcoils with the outer terminal of one/coil connected to the innerterminal of a succeeding coil With the terminals of said choke coilsystem across the inside of one of said choke coils and the outside ofanother of said choke coils for substantially increasing the resis.tance of said choke system at resonant frequencies for preventing theAdestruction of said choke coil system at said resonant frequencies whilemaintaining the distributed capacityy of said choke coil system at a lowvalue.

' ALFRED CROSSLEY.

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